Open sights currently in use on firearms, often as back-up for telescopic sights, typically comprise a fixed but adjustable rearsight located about half way down the action and a fixed foresight which is basically an upstanding rib located towards or at the end of the barrel.
There are a number of problems associated with the use of these sights. The adjustment for the sight is provided on the rearsight, the rearsight tends to become bulky and cumbersome, and can easily interfere with the mounting of telescopic sights on the firearm. In addition the "back-up" nature of these types of sights means that they tend to be miniaturised in an attempt to reduce interference with the telescopic sight, and therefore become fiddly and difficult to use.
In addition, even in cases where the rearsight is retractable it also tends to project from the firearm providing something which can be easily snagged when the firer is clambering over fences and through undergrowth.